1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
With a rapid increase in bandwidth consumption by service subscribers on a wireless network, use of multiple transmit and/or receive antennas has grown considerably across a variety of communication devices and systems. In particular, systems that use multiple transmit and receive antennas are sometimes referred to as multiple-input multiple-output (MIMO) systems. Among other things, for example, a typical MIMO system may deploy multiple transmit and/or receive antennas to increase the achievable bit rate for transmission of large data. However, such use of multiple transmit and/or receive antennas in a MIMO system either involves equalization over large bandwidths or operating over sub-bands, such as in orthogonal frequency-domain multiplexing (OFDM) bands.
A conventional MIMO system that uses multiple antennas processes the channel responses in parallel. When using multiple antennas, a conventional beamformer may steer a beam in different directions before selecting the one that results in the “best” channel response. However, for radio frequency (RF) propagation in a MIMO system, a single channel based on a particular antenna orientation or element phasing may provide an exceptional channel response relative to other available channels. Since a conventional beamformer can only tune to a linear wave-front and does not have the flexibility of arbitrary complex weighting of multiple antennas, significant performance degradation may result.
Moreover, in a diverse scattering environment, a transmitter associated with a MIMO system based on the beam steering may cause a receiver to sense signal energy from many paths over different angles-of-arrival. Even though the beam steering may emphasize some paths over others in such a way as to result in a good channel response, in general, many applications of the beam steering are somewhat limiting since individual performance of antennas of the multiple antennas is based on beam directions. Another approach that uses multiple antennas involves diversity combining to maximize signal power even if the channel response over a desired bandwidth may be unacceptable.
A digital wireless communication system may use either a wideband transmission scheme, such as Code Division Multiple Access (CDMA) or a narrowband transmission scheme, such as orthogonal frequency-division multiplexing (OFDM). A CDMA based wideband transmission may use the channel symbols or chips of a far shorter duration than the maximum delay of the mobile channel. An OFDM based narrowband transmission may use channels (sub-carriers) where many narrowband channels may be transmitted in parallel. The duration of the transmitted symbols over a signal channel may distinguish a wideband wireless digital communication from a narrowband wireless digital communication. For the signal channel, the duration from a first received path to the last received path that has significant power determines the maximum delay of the channel.
However, a radio frequency signal for a wireless digital communication may use multiple paths between a transmitter and a receiver. Such multi-paths may cause signal fading, which may degrade performance of a digital communications system, resulting in lost data or dropped calls in a cellular system. Fading may occur in various forms, including a flat fading. In the flat fading, the same degree of fading takes place for all of the frequency components transmitted through a signal channel and within the channel bandwidth. That is, all the frequency components of the transmitted signal rise and fall together. In contrast, a frequency-selective fading may cause different frequencies of an input signal to be attenuated and phase shifted differently in a channel. An equalizer may provide a desired performance for the channels that may experience frequency-selective fading by restoring the flat fading of the channels. In the time domain, the frequency-selective fading is sometimes called a multi-path delay spread.
One effect of the multi-path fading in the frequency domain is that wideband signals suffer from frequency-selective fading, which means that different parts of the spectrum are faded more than others. Instead, narrowband signals suffer from flat fading where the whole signal spectrum fades. However, if an antenna receives a signal with zero amplitude, the signal is sometimes referred to be in deep fade. Deep fades occur more frequently the faster a mobile station travels, but the duration that the signal is in deep fades decreases as the speed of the mobile increases.
Additionally, a conventional equalizer may use time delay taps. A time delay tap provides a setting with adjustable time delay intervals to trip after a set delay regardless of an input signal state. By varying the delay time tap setting, an equalizer may define handling of transients. However, this use of time delay taps for equalization of a signal propagation channel cannot accommodate deep fades without sacrificing noise immunity.